(282a) Compressor-Free Heat Integrated Distillation Column System for Dehydration of Fermented Mash In Production of Fuel Bio-Ethanol

This paper reports a simulation analysis of energy consumption about a newly-invented heat integrated distillation column (named “Compressor-free HIDiC”) system for production of fuel ethanol from the fermented mash. The cost performance for lowering the fuel ethanol price depends upon the energy-saving technology of dehydration process. The original HIDiC system proposed by Mah et al. has a compressor for raising the boiling point of the rectifying section. This system cannot be applied for the process owing to the fouling problems coming from fermentation residues and dissolved inorganic salts. The new system consists of an ordinary evaporator-like column (1st column, i.e. mash or beer column) and a double-tube heat integrated distillation column (2nd column) without a compressor. The second column has a normal-pressure rectifying section inside the inner tube and a low-pressure stripping section in the annular space outside the inner tube. Both sections are installed with original dual-flow trays named “Lift tray”. The overhead vapor issuing from the top of the mash column operated at normal pressure is directly supplied without a compressor into the bottom of the rectifying section operated at a normal or a little bit lower pressure. Another distinctive feature is that the stripping section has an overhead condenser, as different from the original HIDiC system. The condensate from the overhead condenser is partially refluxed to its own top of the stripping section and partially fed back into the top of the mash column as a kind of external reflux.

The target of dehydration from 6 wt% ethanol solution (fermented mash) is that the overhead product of the rectifying section should have an approximately 88 wt% of ethanol suppressed below the azeotropic composition (95 wt%).

The following special problems arise owing to the environmental circumstances of sugar cane island:

(1) The waste water as the final bottom product should not contain above 100 ppm of ethanol,

(2) The residual molasses after sugar production should be employed as the feed for ethanol fermentation. Therefore not only bioprocess residues but also various inorganic salts are contained in the fermented mash even after filtration. Especially much calcium sulfate deposits on the trays of distillation column if the tray temperature is raised and the perforated trays below the feed stage are very often stopped up.

The above problems are based on actual experience in a governmental project carried out in a coral island producing sugar cane.

The mash column should have a very simple inside structure with a very small number of trays for the purpose of maintenance. Therefore the bottom product from the mash column reboiler should be supplied into the top of the stripping section of the second column to lower the bottom product concentration of ethanol below 100 ppm.

The new HIDiC system proposed can solve both problems by supplying the bottom product of the mash column into the top of the stripping section of the second column after filtration and by depressurizing the stripping section to lower the tray temperature in order for calcium sulfate not to deposit.

The simulation analysis has been made by varying the operation pressure of the second column stripping section and the issuing rate of the overhead vapor from the mash column as the control parameters.

It has been indicated that if the HIDiC stripping section is depressurized to 200 mmHg, a very high energy-saving rate above 50% in comparison with an ordinary bio-ethanol dehydration plant can be attained with the new HIDiC system under the same separation conditions.